As a microwave plasma torch capable of generating plasma in atmospheric pressure, there is conventionally known a waveguide microwave plasma torch (see Patent Document No. 1). This conventional waveguide microwave plasma torch roughly includes three components: a stub tuner, a waveguide and a reflecting plate, and the torch further requires an ignition device for generating plasma in atmospheric pressure and thus has a large number of components. Hence the conventional waveguide microwave plasma torch has a problem of having a low degree of flexibility in device design to limit an attempt to downsize the device.
As a plasma torch to solve this drawback of the conventional waveguide plasma torch, there is proposed a coaxial microwave plasma torch having a configuration inherited from a configuration of a helical resonator (see Patent Document No. 2). This microwave plasma torch has a coaxial resonator including a cylindrical outer tube having an upper-end opening closed with a lid, and a coaxial line coupled at right angles to the outer tube of the resonator at a part closer to the upper end. The conductor passing through the inner center of the coaxial line is bent upward in a direction toward the lid inside the outer tube to be fixed to the inner end face of the lid, the lid is connected to an outside conductor of the coaxial line through the outer tube, an inside conductor is fixed to the center of the lid, the inside conductor includes a stick section and an electrode which has an electric conductivity and is fixed to the top of the stick section, a silica tube is fitted to the peripheral face of the electrode, and a gas inlet for guiding gas from the outside toward the electrode is provided in the peripheral wall of the outer tube.
In this microwave plasma torch, when a microwave is outputted with a microwave oscillator connected to the coaxial line, the microwave is allowed to pass through the coaxial line to be converted into a coaxial mode (TEM mode) and then transmitted. Subsequently, the microwave is once mode-converted at the section where the conductor passing through the inner center of the coaxial line is bent in a direction toward the lid inside the outer tube of the oscillator, and converted again into the coaxial mode inside the oscillator to be led to the electrode by the inside conductor, whereby electric fields of the microwave are concentrated at the top of the electrode to maximize the electric field intensity, and a plasma is thus generated from the top of the electrode.
However, according to this configuration, the use of the oscillator has required the plasma torch to be kept in a certain degree of size, making it difficult to downsize the plasma torch. Further, according to this configuration, the microwave is once converted from a coaxial mode into another mode and then again converted into the coaxial mode during transmission of the microwave from the coaxial line into the oscillator, but there has been a problem that, when such mode conversion is performed, energy loss occurs in response to the conversion, thereby decreasing an energy efficiency. In addition, according to this configuration, it has been difficult to ignite plasma in atmospheric pressure.    Patent Document No. 1: Japanese Laid-Open Patent Publication No. H9-295900.    Patent Document No. 2: Japanese Laid-Open Patent Publication No. H6-188094.